No probate is necessary for Christian and Muslim Wills.
However the religion of the testator is not known, hence in the absence of probate of Will the consistency or enforceability of the Will cannot be determined.
Whenever there is a proof of testamentary succession, the genuineness of the instrument through the succession has been described has to be examined/scrutinised properly, which will include the clear and marketable title the testator towrds the property bequeathed through the Will.
After that, to enforce the Will the beneficiaries have to get the property revenue or mutation records transferred to their names from that of the testator by submitting an application before the revenue department.
In the revenue department it would be pertinent to submit NOC from other legal heirs, hence the prospective buyer may insist the sellers to get the property transferred to their names first before executing the registered sale deed.
This process would clarify and confirm the other issues that have been raised by you.
Therefore it is always advisable to obtain a proper legal opinion from an experienced local lawyer before venturing into the proposed purchase of immovable property.